1. Field of the Invention
The present invention relates to a DC motor controller such as a DC motor control circuit which can rotate a DC motor in a desired one of the forward and reverse directions and stop the rotating DC motor at a desired time (position) and which is suitably used for driving an actuator for changing the direction of a parabolic antenna for satellite broadcasting reception.
2. Description of the Related Art
In general, a DC motor can be easily handled in comparison with an AC motor and is frequently used since it is easy to change the rotation direction thereof. For example, in a satellite broadcasting reception system used in the United States of America, an actuator which is driven by a DC motor is mounted on a parabolic antenna (which is generally called a dish) and is constructed to be remotely controlled on the satellite broadcasting receiving side so as to automatically change the direction of the parabolic antenna towards a desired one of a plurality of satellites.
The conventional DC motor controller used for changing the direction of the parabolic antenna is shown in FIG. 6.
In FIG. 6, 1 denotes a power source transformer which has a primary winding L1 connected to receive an AC voltage from a commercial AC power source 2 and a secondary winding L2 connected to a rectifier 3. The rectifier 3 applies a DC voltage appearing between positive and negative voltage terminals P1 and P2 to a DC motor 13 via relays 7 and 11.
That is, the relay 7 is constructed by two-contact change-over switches 5 and 6 and a coil L7 and the relay 11 is constructed by two-contact change-over switches 9 and 10 and a coil L11. The contacts a of the switches 5 and 6 are respectively connected to the terminals P1 and P2 and the contacts b thereof are connected together. The contacts a of the switches 9 and 10 are respectively connected to the negative and positive voltage terminals P2 and P1 of the rectifier 3 and the contacts b thereof are commonly connected to a common contact c of the switches 5 and 6. Further, the common contacts c of the switches 9 and 10 are respectively connected to the positive and negative polarity terminals of the DC motor 13 via terminals P3 and P4. The coil L7 is connected between a bias terminal 4 to which a preset voltage V is applied and the collector of a transistor Q1 and the coil L11 is connected between the preset bias terminal 4 and the collector of a transistor Q2.
The transistors Q1 and Q2 are used for controlling the relays 7 and 11 and have emitters connected to the ground terminal and bases supplied with control signals from terminals 8 and 12.
The relays 7 and 11 change the polarity of a voltage from the rectifier 3 and then supplies the same to the DC motor 13 so that the DC motor 13 can be controlled to rotate in a forward or reverse direction.
With the above DC motor controller, when the DC motor 13 is rotated in a forward direction, a control signal supplied from a control circuit (not shown) to the control signal input terminal 8 is set to a high level and a control signal supplied from the control circuit to the control signal input terminal 12 is set to a low level. As a result, a current flows in the coil L7, no current is permitted to flow in the coil L11, the contacts a and c of the switches 5 and 6 are switches 9 and 10 are connected together. Therefore, the DC motor 13 is applied with positive and negative voltages at the positive and negative terminals, respectively. As a result, the DC motor 13 rotates in the forward direction.
In a case wherein the DC motor 13 which is rotating in the forward direction is stopped at a desired time (position), the control signal supplied to the control signal input terminal 8 is changed to a low level while the control signal supplied to the control signal input terminal 12 is kept at the low level. Then, no current is permitted to flow in the coils L7 and L11, and the contacts b and c of each of the switches 5, 6, 9 and 10 are connected together. As a result, the positive and negative terminals of the DC motor 13 are shortcircuited and a current caused by a counterelectromotive force flows in the DC motor 13 in a direction opposite to that of the current flowing at the time of forward rotation, thus instantaneously interrupting the rotation of the DC motor 13. This is because a braking force acts in such a manner that an inertia force in the forward rotating direction of the DC motor 13 can be cancelled by a driving force in the reverse rotating direction, and this explanation is omitted in the following description.
When the DC motor 13 is rotated in the reverse direction (which is hereinafter referred to as "reversely rotated"), a control signal supplied to the control signal input terminal 8 is set to the low level and a control signal supplied to the control signal input terminal 12 is set to the high level. As a result, a current flows in the coil L11, no current is permitted to flow in the coil L7, the contacts b and c of the switches 5 and 6 are connected together and the contacts a and c of the switches 9 and 10 are connected together. Therefore, negative and positive voltages are respectively applied to the positive and negative terminals of the DC motor 13, thereby reversely rotating the DC motor 13.
In a case wherein the DC motor 13 which is reversely rotated is stopped at a desired time (position), the control signal supplied to the control signal input terminal 12 is changed to the low level while the control signal supplied to the control signal input terminal 8 is kept at the low level. Then, the positive and negative terminals of the DC motor 13 are short-circuited in the same manner as in the case wherein the forwardly rotating motor 13 is stopped, and a current caused by a counter-electromotive force flows in the DC motor 13 in a direction opposite to that of the current flowing at the time of reverse rotation, thus instantaneously interrupting the rotation of the DC motor 13.
The conventional DC motor controller effects the rotation direction changing operation and rotation stopping operation by use of the relays 7 and 11. The relay 7 is constructed by the switches 5 and 6 and the relay 11 is constructed by the switches 9 and 10. Each of the switches 5, 6, 9 and 10 has two electrical contacts and therefore the DC motor controller has eight electrical contacts in total. Since the conventional DC motor controller has such a large number of electrical contacts, it tends to be damaged and the reliability thereof is low. The contacts are used for turning ON and OFF the direct current and are formed of expensive alloy (for example, silver-gold alloy). Therefore, the manufacturing cost thereof is high.
In short, the conventional DC motor controller has two relays each formed of two switches in order to rotate the DC motor in the forward and reverse directions and short-circuit the DC motor. Further, each of the switches has two electrical contacts and therefore eight electrical contacts in total are provided in the DC motor controller, thus causing malfunctions to occur frequently and lowering the reliability thereof.
That is, the above problems may occur because the relay contacts for turning ON and OFF the DC circuit permit a current to flow only in one direction and the contacts which will be melted by arcs occurring at the time of ON and OFF times are unconditionally determined.